1. Field of the Invention
This invention relates to a method of manufacturing a grain-oriented electrical steel sheet, which is primarily used as an iron core material for large-sized motors, generators and transformers, which does not have an undercoating made of primarily forsterite (Mg2SiO4) (glass coating), and has a high magnetic flux density and preferably has a low iron loss.
2. Description of the Related Art
Grain-oriented electrical steel sheets having a low iron loss are used as iron core material for large-sized motors, generators and transformers because energy loss attributable to iron loss is considered as an important factor in such equipment.
FIG. 1 shows, by way of example, the shape of punched pieces of a grain-oriented electric steel sheet, which are laminated to form an iron core (stator) of a large-sized generator. As shown in FIG. 1, a number of fan-shaped segments 2 are punched from a grain-oriented electrical steel sheet 1 supplied in the form of a strip, and the iron core is assembled by laminating the segments 2 one above another.
When employing such a laminating method, each segment is punched into a complicated shape including teeth 3.
Also, dies are employed to punch several tons or more of iron core material, and a very large number of times of punching is required. Therefore, a grain-oriented electrical steel sheet causing less wear of the dies when punched successively, namely, having good punching quality, is demanded.
Surfaces of a grain-oriented electrical steel sheet are usually coated with an undercoating made of primarily forsterite (Mg2SiO4) (glass coating). Undercoating made of primarily forsterite strongly adheres with the coating thereon (usually comprising phosphate and colloidal SiO2), so that said coating thereon can apply tension to the steel sheet. Because the tension applied to steel sheet reduces the iron loss of the steel, undercoating made of primarily forsterite is substantially necessary to ensure excellent magnetic characteristics. However, because the forsterite coating is much harder than a coating of an organic resin that is coated on a non-oriented electrical steel sheet, wear of the punching dies is increased. Accordingly, re-polishing or replacement of the dies is required at higher frequency, which reduces the work efficiency and increases the cost when iron cores are manufactured by iron-consuming makers. Further, slitting and cutting quality are similarly deteriorated by the presence of the forsterite coating.
As a method of improving punching quality of a grain-oriented electrical steel sheet, it is conceivable to remove the forsterite coating by pickling or a mechanical manner. However, this method not only increases the cost, but also raises a serious problem that the surface of the steel sheet is marred and magnetic characteristics are deteriorated.
Japanese Examined Patent Application Publication Nos. 6-49948 and 6-49949 propose a technique for inhibiting formation of the forsterite coating by mixing an inhibitor in an annealing separator that is made of primarily MgO and is applied in a final finishing annealing step. Additionally, Japanese Unexamined Patent Application Publication No. 8-134542 proposes a technique for applying an annealing separator, which is made primarily of silica and alumina, to a starting material containing Mn.
With those proposed techniques, however, it is very difficult to obtain a product sheet in which generation of forsterite is completely inhibited, because forsterite is partly formed in many cases with local variations in the final finishing annealing atmosphere caused between coil layers.
In view of that situation, we previously proposed, in Japanese Unexamined Patent Application Publication No. 2000-129356, a technique for developing secondary recrystallization in a high-purity material, which contains no inhibitor component, by utilizing the grain boundary migration suppressing effect of solid solution nitrogen. Also, we previously proposed, in Japanese Unexamined Patent Application Publication No. 2001-32021, a technique for suppressing generation of an oxide coating by using a composition containing a reduced amount of C and by low-oxidation atmosphere for recrystallization annealing has less oxidizing power.
Those techniques succeeded in manufacturing a grain-oriented electrical steel sheet in which forsterite is not formed at a relatively inexpensive cost. The thus-manufactured grain-oriented electrical steel sheet is suitably used for large-sized motors and generators in which punching quality is important, because the steel sheet has no hard forsterite coatings on its surfaces.
However, when manufacturing a grain-oriented electrical steel sheet without using an inhibitor, there still remains the problem that the manufactured steel sheet has a lower magnetic flux density than the case of manufacturing it using an inhibitor.
With the view of effectively overcoming the problem set forth above, it would be advantageous to provide a novel manufacturing method which can advantageously manufacture a grain-oriented electrical steel sheet having a sufficiently high magnetic flux density and preferably having a low iron loss, even when no inhibitor is used in the manufacturing process.
It is to be noted that this invention is also applicable to the case of manufacturing a grain-oriented electrical steel sheet using an inhibitor and can advantageously manufacture a grain-oriented electrical steel sheet having a sufficiently high magnetic flux density and a low iron loss.
As a result of conducting intensive studies to achieve the above object, we discovered that, when manufacturing a grain-oriented electrical steel sheet not having a forsterite coating by using a starting material which contains no inhibitor component, the magnetic flux density is remarkably improved by performing final finishing annealing (secondary recrystallization annealing) in the state where a certain amount of C remains, and that magnetic characteristics are further remarkably improved by additionally performing high-temperature continuous or batch annealing in a non-oxidizative or low-oxidizative atmosphere after decarburization annealing. Further, we discovered that the secondary recrystallization annealing is able to serve also as decarburization annealing by introducing a hydrogen atmosphere during the second-half period of the annealing process at high temperature.
Thus, selected features of the present invention are as follows:
The invention resides in a method of manufacturing a grain-oriented electrical steel sheet not having an undercoating made of primarily forsterite (Mg2SiO4) and having a high magnetic flux density, the method comprising the steps of preparing a slab using molten steel containing, by mass %, C of not more than about 0.08%, Si of about 1.0 to about 8.0% and Mn of about 0.005 to about 3.0%, in which the contents of Al and N are preferably reduced to be not more than about 150 mass ppm and about 50 mass ppm, respectively; rolling the slab to obtain a steel sheet; performing primary recrystallization annealing (so-called xe2x80x9crecrystallization annealingxe2x80x9d) on the rolled steel sheet in an atmosphere with the dew point of preferably not higher than about 40xc2x0 C. and adjusting the C content in the steel sheet after the primary recrystallization annealing to be held in the range of about 0.005 to about 0.025 mass %; performing secondary recrystallization annealing (so-called xe2x80x9cfinal finishing annealingxe2x80x9d, usually batch annealing) in an atmosphere with the dew point of preferably not higher than about 0xc2x0 C.; and then performing decarburization annealing.
In the above-described method, preferably, the rolling step comprises steps of hot-rolling the slab; annealing a hot-rolled sheet as required; and performing cold rolling once, or twice or more with intermediate annealing therebetween.
In the above-described method, the secondary recrystallization annealing is preferably performed without applying an annealing separator, but the secondary recrystallization annealing may be performed after applying an annealing separator that does not form forsterite (i.e., does not contain MgO).
In the above-described method, preferably, the secondary recrystallization annealing is performed in a nitrogen-containing atmosphere.
Also, for obtaining a grain-oriented electrical steel sheet having a high magnetic flux density and a low iron loss, molten steel containing Al in amount reduced to be not more than about 100 mass ppm, and N, S and Se in amounts each reduced to be not more than about 50 mass ppm is used as the aforesaid molten steel.
Further, preferably, the molten steel (or the steel sheet) contains, by mass %, at least one element selected from among Ni: about 0.01 to about 1.50%, Sn: about 0.01 to about 0.50%, Sb: about 0.005 to about 0.50%, Cu: about 0.01 to about 0.50%, P: about 0.005 to about 0.50%, and Cr: about 0.01 to about 1.50%.
The C content in the molten steel is preferably not less than about 0.005 mass %, and preferably not more than about 0.025 mass %.
In the above-described method, the decarburization annealing is preferably performed as continuous annealing in a humid atmosphere. As an alternative, flattening annealing serving also as the decarburization annealing may be performed.
Also, in the process of manufacturing a grain-oriented electrical steel sheet having a high magnetic flux density and a low iron loss, the steel sheet may be decarburized in the second half of the secondary recrystallization annealing instead of performing the decarburization annealing as a separate step. When decarburizing the steel sheet in the second half of the secondary recrystallization annealing, a hydrogen atmosphere with a partial pressure of not lower than about 10 volume % is preferably introduced and the temperature range is preferably not lower than about 900xc2x0 C. during the secondary recrystallization annealing. In that case, preferably, heat treatment is performed in the temperature range of about 800 to about 900xc2x0 C. for about 300 minutes or longer before introducing the hydrogen atmosphere.
Moreover, preferably, the C content is reduced to be less than about 50 mass ppm with the decarburization annealing.
Preferably, after performing the decarburization annealing in a humid atmosphere subsequent to the secondary recrystallization annealing, continuous annealing (called xe2x80x9cadditional continuous annealingxe2x80x9d) for holding the steel sheet to reside in the temperature range of not lower than about 800xc2x0 C. for at least about 10 seconds is performed in an atmosphere with the dew point of not higher than about 40xc2x0 C. With this process, a grain-oriented electrical steel sheet having further improved magnetic characteristics, a higher magnetic flux density and a lower iron loss can be obtained.
Alternatively, preferably, after performing the decarburization annealing in a humid atmosphere subsequent to the secondary recrystallization annealing, batch annealing (called xe2x80x9cadditional batch annealingxe2x80x9d) for holding the steel sheet to reside in the temperature range of about 800 to about 1050xc2x0 C. for at least about 5 hours is performed in an atmosphere with the dew point of not higher than about 40xc2x0 C. With this process, a grain-oriented electrical steel sheet having further improved magnetic characteristics, a higher magnetic flux density and a lower iron loss can be obtained.
Prior to the additional batch annealing, an annealing separator not forming forsterite (i.e., not containing MgO) may be applied as required.